Lithium ion secondary batteries (referred to also as “Li batteries” hereinafter) have a high energy density to be used for mobile instruments such as portable telephones. Moreover, by making use of properties thereof, the development of large-sized batteries, for example, for being mounted in cars has also been advanced. In the meantime, Li batteries have various themes. Among these themes, an important theme is to heighten the capacity of the batteries. In recent years, about mobiles instruments, smartphones increase rapidly. Smartphones are large in electric power consumption to be usable only in a short period under the present circumstances. It is therefore essential to heighten the batteries in capacity. Furthermore, about large-sized batteries for being mounted in cars and others, the distance that the batteries can cause the cars and the others to run is short; thus, in order to make this distance longer, high-capacity batteries are required as well.
For an increase in the capacity of Li batteries, the development of high-capacity materials has been advanced. About a negative electrode of a Li battery, attention has been paid to negative electrode active material (negative electrode material) in which silicon (Si) is used. Si has a theoretical capacity of 4200 mAh/g, which is 10 or more times larger than that of carbon, which has been mainly used at present; thus, Si is expected as a high-capacity material. Additionally, the following have been paid as capacity-heightened materials: SiOx, which is produced by mixing Si with silicon dioxide (SiO2), and SiOx-carbon-complexed-material (SiOxC), in which SiOx is covered with carbon.
However, these Si based negative electrode active materials are high in capacity while the materials have a room for a further improvement of cycle characteristic in accordance with a high-level request. Thus, a technique is developed in which fluorinated ethylene carbonate (FEC) is added to an electrolytic solution to improve the cycle characteristic of a Si based negative electrode active material (Patent Literature 1). By the addition of FFC, a covering layer is formed on a negative electrode, this layer having an ion conductivity and further blocking the electrolytic solution. The formation of the covering layer would produce the following advantage: the co-insertion of a solvent is prevented, the insertion being generated when the negative electrode occludes Li, and the Si based negative electrode active material is heightened in structural stability, so that the cycle characteristic is improved.